1. Field of the Invention
The present invention relates to the structure of a fixed-focus amphibious camera and, more particularly, to a camera capable of bringing any subject to be photographed into focus both on land and in water.
2. Description of the Prior Art
When photographs are taken in water, a camera specifically designed for use in water and having water-proof structure or a camera housed in a waterproof casing is used. However, many limitations are imposed on photographing in water, unlike photographing on land. First, light is attenuated by water, reducing the maximum object distance available. Even in water having a good transparency, the maximum object distance that can be achieved is as short as about 25 m. Also, as deeper it goes, made shorter the object distance. Further, if many objects or materials float in water, the distance is further reduced, and the contrast is lowered. In addition, water selectively absorbs light components of longer wavelengths, so that the light coming from a subject to be photographed is tinted blue. Furthermore, the light emitted from a subject located remotely becomes totally blue. Accordingly, when clear photographs are to be taken in water, the camera is urged to be set closer to the subject than on land. Thus, a predetermined object distance at which photographs are taken most frequently is of the order of 2 m on land, whereas, in water, the object distance is approximately 1.5 m and photographs are often taken even at still shorter distances. Underwater photographing often employs flashlight on account of low luminance of subjects to be photographed, but the flashlight is absorbed by water. For this reason, photographs must be taken at shorter object distances. Consequently, cameras for use in water are required to be so designed that subjects are brought to focus at shorter distances than on land. As such, a fixed-focus amphibious camera is so designed that for use on land, subjects are brought to focus at an object distance predetermined for photographing on land and for use in water, subjects are brought to focus at a shorter object distance predetermined for photographing in water. However, the prior art fixed-focus amphibious camera is so made that subjects are brought to focus at an object distance predetermined either for on land or in water resulting in a possibility that photographs are taken which are out of focus. For example, if a camera adapted to take photographs on land is used to take photographs in water, subjects are photographed at too short distances, resulting in photographs that are out of focus. In the reverse situation, photographs are taken at too large distance on land, producing photographs that are out of focus.
A camera which is so designed that a subject at a suitable distance is brought to focus in water is disclosed in U.S. Pat. No. 4470680. This camera is equipped with a device to sense whether it is on land or in water. When the camera is put into water, the object distance which is brought into focus is automatically switched to the value preset for photographing in water.
However, this camera requires a sensor device making use of photoelectric devices, for example, thus making the structure complicated. This increases the cost to manufacture the camera.
Another camera is disclosed in Japanese Patent Laid-open Publicity No. Sho. 54-85721, in which an attachment lens is installed in front of the lens system to permit any subject at the same distance to be brought to focus with exactly the same adjustment of focus whichever the camera is on land or in water. This camera utilizes the difference between the refraction of light occurring at the boundary between the lens and water and the refraction of light occurring at the boundary between the lens and air. The difference is caused by the difference between the indices of refraction of water and air. However, this camera is capable of adjusting focus. In water, the distance to the subject is actually measured with a metering device. The focus is made to conform with this measured value.
Although this technique using an attachment lens might be similarly adapted to a fixed focus camera, the result would be such that objects located at the same distance are brought into focus whether they are on land or in water, thus leaving the foregoing difficulties unsolved.